The invention relates to an arrangement for coupling light. In particular, it relates to an arrangement for coupling light of a semiconductor laser diode into a multimode glass fiber.
If light signals are emitted by a laser diode and directed through a multimode glass fiber, they are normally distorted by noise components. This noise originates at dissipated plug connects, which are unavoidable due to fluctuations in the light intensity distribution in the fiber core. This problem is usually referred to as "modal noise". Its cause is the unstable interference of the coherently excited fiber modes having phase velocities which are different and fluctuate significantly in the optical spectrum. Phase velocity fluctuations are caused, for example, by mechanical vibrations of the fiber and by fluctuations in the optical emission spectrum of the laser diode. The light intensity noise varies less the less connector loss and the greater the number of excited fiber modes and spectral laser emission modes. Operationally stable 1.3 micrometer semiconductor laser diodes, however, have narrow line widths (smaller than 5 nanometer) with only a few emission modes. The currently used fiber couplings are highly efficient, but excite only a few fiber modes. This situations is especially unfavorable with respect to the above described light intensity noise in multimode fiber transmission lines. If a multimode lightwave guide is used for long distance transmissions, even in digital transmission systems (for example at 140 megabytes per second) the operation is occassionally disturbed, in spite of low attenuation values 1 dB of the used plug connections.
Attempts at remedying this have been made using specific light emitters or fibers. So, gain guided laser diodes with low coherence and a multitude of spectral emission modes, respectively, (GaAs/GaAlAs oxide strip lasers) are employed for a 850 nanometer operating wavelength. Or, the fiber mode characteristic is improved by using fiber mode mixers. Such mixers are, however, mechanically elaborate and, to our knowledge, have not yet been used in practice. In fact, all these attempts have failed, so far, to provide a setting which ensures a stable operation under all conditions. This situation can partly be attributed to the fact that an additional reason for instability, namely the distortions created by light reflected from the fiber back into the laser, has not properly been taken into account.